The SEC is set to propose a rule change tomorrow that, if adopted, will transform the fierce rivalry between the nation’s two most significant marketplaces – the New York Stock Exchange and electronic giant Nasdaq – into full-blown corporate warfare.

Already intense competitors, the two exchanges have furiously lobbied Congress and the SEC to endorse their own positions on the rule change.

The NYSE has wanted to leave intact the so-called trade-through rule, which requires that trades be executed at the best price – a condition that favors the Big Board.

Nasdaq and its electronic trading rivals, meanwhile, have called for the rule’s abolition. “The trade-through rule is an anachronism today and we believe that it should be repealed,” Nasdaq CEO Robert Greifeld wrote in his testimony before a congressional hearing Friday.

Both will likely be disappointed.

The commission is expected to vote unanimously to put forth the proposed change – which will allow institutional traders to opt out of the trade-through rule, which primarily governs trading of NYSE stocks, but will apply the rule to Nasdaq-listed stocks as well.

The rule, if approved by the commission, would potentially drive down the New York Stock Exchange’s 80 percent market share and allow Big Board competitors to gain more than their current 20 percent share of trading in NYSE stocks.

But the new change would require the Nasdaq, which has never had a trade-through rule, to abide by the new one.

It’s unclear what the impact of that will be – except that the NYSE and Nasdaq, as well as competitors like ArcaEx and Instinet, will be wooing customers on a trade-by-trade basis, creating more competition in an already competitive landscape.